Medicare Supplement Insurance:

Although Original Medicare (Part A and Part B) pays for some preventive services and covers most medically necessary services, but it does not cover all medical services. Also it does not pay 100% of covered services. There are deductibles, co-pays ans co-insurance.

Some of the services not covered by Original Medicare are hearing aids, eyeglasses, dental care, acupuncture, chiropractic care, long-term care at home or in a nursing home and health care outside the United States.
With only Original Medicare you could have sizable out-of-pocket health care expenses and they will increase as you age.

Medicare Supplement Insurance, sold by private insurance companies, helps cover expenses Medicare does not cover - deductibles, co-pays, and co-insurance. You must be enrolled in both Medicare Part A and Part B to purchase a Medicare Supplement insurance plan.